Introduction to Genetics

Hi Faith,I'm going to take a shot at answering the questions about chromosomes, but I'm going to keep it simpler than most have. Keep in mind that there can be exceptions to anything, but by and large what I'm going to tell you will be true 99 and 44/100% of the time. Each species has a fixed number of chromosomes that is the same for all individuals. The number of chromosomes can vary from 1 up to 132 (the highest figure I happened to find) and beyond, but probably very few species have over a hundred chromosomes. Since there are millions of species naturally many species will have the same number of chromosomes, but that's a matter of coincidence and is of no consequence. The number of chromosomes has little effect on what a species looks like.If someone gave you two cells from two different species and you found that they had the same number of chromosomes, simply assuming they're the same species would be a big mistake.Each chromosome is a long tightly wound tangle of DNA. Coded within the DNA are the genes. A specific gene always resides on the same chromosome, though it can move around somewhat on that chromosome during reproduction.--Percy

Yes, someone did say this, and I have no idea why. While it is possible, it's potential for confusion makes it something that shouldn't be mentioned when merely introducing genetic concepts. I agree. I think you're asking a great question: what is the advantage of dividing a cell's DNA into separate pieces we call chromosomes. Presumably it came about because it provides some advantage (error reduction possibly?) Maybe someone out there knows the answer. It is genes that code for the proteins that produce different functions. Each gene is comprised of a part of the length of the DNA strand that makes up a chromosome. Usually the coiled DNA strand that is a chromosome contains many genes. Checking with Wikipedia, there are many genes that play a role in determination of eye color. Choosing just one as an example, OCA2, for humans it always resides on chromosome 15, and almost always in the same place on chromosome 15. I wasn't able to find any information about where it is in other species. Rather a lot.--Percy

If you're asking how you look at a set of chromosomes and tell that this one is #12 and that one is #17, I don't know.--Percy

For the most part, genes do not change chromosomes. For example, for almost every human being on the planet gene OCA2 lies on chromosome 15. But the copying and combining of DNA that occurs for sexual reproduction is not perfect, and as Crash says, anything can happen. There no doubt are some human beings for whom the OCA2 gene does not lie chromosome 15, but genetic accidents like this shouldn't be of interest in an introductory course. Gene swapping between chromosomes is rare. Extremely rare. The "jumping genes" that have been mentioned a couple times change location on a chromosome, not between chromosomes. Jumping between chromosomes is not impossible, but it doesn't happen very often. We can give the label Crashfrog's Law to the fact that things that are not impossible will happen at least occasionally. This is a good question that involves the mechanics of protein production and the copying of reproduction. I'd be interested in the answer myself.--Percy